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The results of three types of refinement are presented inthis paper. In the first series of refinements, the symmetry ofthe sample was cubic (Fm3m) and the profile function wasisotropic in crystal space. In the second series, the symmetryremained cubic but the profile function of Stephens was usedto fit the observed peak shapes, which were found to beanisotropic in crystal space. In a third series of refinements,a tetragonal distortion was allowed (I4/mmm) wheneversignificant line broadening was observed, but the peak-shapes were isotropic in crystal space.III. Results1. Observed diffraction patternsWe did not observe the expected a'-phase diffractionpattern on the first cooling to low temperatures, but only thediffraction pattern of 6-phase alloy. The diffraction patternsthat we observed are shown in Fig. 1.Because the diffraction pattern of the monoclinic alphaphase has many weak diffraction peaks, we make a specialRietveld analysis of the low temperature data that included10 at. % of the a'-phase. We estimated the lattice constantsof a' at low temperature by including corrections for theanisotropic expansion observed as a - a' at roomtemperature under pressure and the anisotropic thermalcontraction observed as a is cooled to low temperatures. Theestimated lattice constants for a'-Pu at 15K are a=6.142,b=4.797 and c=10.915A with 3=101.850. In order toenhance the visibility of weak diffraction peaks, the datawere compressed by a factor of 8 along the d-spacing axis.

0(NCa0NEu 00z

0.5d-spacing (A)

1.0

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00.0

0.5 1.0 1.5d-spacing (A)

2.0 2.5 3.0

Fig. 1. Neutron diffraction patterns from NPD of Pu 2 at. %Ga at 300K (new sample) and 15K (first cool). Theobserved strong reflections are from 6-phase Pu.There are a few weak reflections for the vanadiumcontainment